Selective catalyst dehydrogenation of a mixture of normal paraffin hydrocarbons



United States Patent 0.

I US. Cl. 260-683.3 8 Claims ABSTRACT OF THE DISCLOSURE A mixture of at least four normal paraflin homologs is selectively dehydrogenated to produce normal monoolefin analogues of the normal paraffins by fractionating the mixture into a plurality of fractions each of which contains at least one of the normal paraflin homologs, separately dehydrogenating each of the resulting fractions with a nonacid, platinum metal-containing, aluminasupported catalyst, and regulating the severity level employed with each fraction such that the severity level varies inversely with the average carbon number of the normal para-flins contained therein. Key feature of the resulting process is its high selectivity for production of normal monoolefins are compared with the selectivity achieved when the entire mixture is simultaneously subjected to dehydrogenation conditions in the presence of this catalyst.

DISCLOSURE The subject of the present invention is an improved process for the selective dehydrogenation of a mixture of at least four normal paraffin homologs to produce normal monoolefin analogues of these normal paraffins. More specifically, the present invention comprehends a method of improving the selectivity and stability characteristics of a catalytic dehydrogenation process which utilizes a nonacid, alumina-supported, platinum metalcontaining catalyst to transform a mixture of normal paraffin hydrocarbons to the corresponding normal monoolefins with minimum production of side products.

The concept of the present invention developed from my investigations of the problems associated with the charging of a wide-boiling feed stream to a catalytic dehydrogenation process which has been specifically designed to have a high selectivity and stability for the production of long-chain normal monoolefins. The principal problem encountered essentially involves the degradation of these selectivity and stability parameters relative to the values experienced with a feed stream boiling in a narrow range. Quite unexpectedly, I have now found a method that allows the recapture of these high selectivity and stability features of this process when a wide-boiling range feed stream is charged thereto. Essentially, my method involves fractionating the wide boiling feed stream into a plurality of fractions and separately dehydrogenating each of the fractions at severity levels which are inversely related to the average carbon number of the normal paraflins contained therein.

Although extensive work has been done in the general area of production of monoolefins from parafiins, the chief effort in the past has been primarily concentrated on lower molecular weight paraflins (i.e. paraffins having 2 to 6 carbon atoms). This concentration of effort was basically caused by the ready availability of large quantitles of these para-flins coupled with the building-block nature of the product olefins that could be made therefrom. Recently, attention within the chemical and petroleum industries has been focused upon the problem of 3,474,156 Patented Oct. 21, 1969 acquiring long-chain, normal monoolefins. In particular, a substantial demand has been established for normal monoolefins having 6 to 20 carbon atoms. As might be expected, this demand is primarily a consequence of the growing commercial importance of the products that can be synthesized from these normal monoolefins. For example, derivatives of normal monoolefins have become of substantial importance to the detergent industry because these normal monoolefins can be used to alkylate an alkylatable aromatic, such as benzene, and the resultant arylalkane can be transformed into a wide variety of biodegradable detergents such as the alkylaryl sulfonate (anionic) type of detergent which is most widely used for household, commercial and industrial purposes. An other type of detergent produced from these arylalkanes is aJkylaryl-poilyoxyalkylated amines. Still another large class of the detergents produced from these normal monoolefins are the oxyalkylated phenol derivatives in which the alkyl-phenol base is prepared by the alkylation of phenol. Other uses of these long-chain monoolefins include: direct sulfation to form biodegradable alkyl sulfates of the type R--OSO Na; direct sulfonation with sodium bisulfiite to make biodegradable sulfonates of the type R-SO Na; hydration to alcohols which are used to produce plasticizers or synthetic lube oils; hydration to produce alcohols followed by dehydrogenation to form ketones which can be used in the manufacture of secondary amines by reductive alkylation; ester formation by direct reaction with acids in the presence of catalysts such as BF -etherate; and the preparation of di-long chain alkylbenzenes of which the heavy metal sulfonates are prime lube oil detergents.

Responsive to this demand for long-chain, normal monoolefins, the art has developed a number of alternative methods to produce them in commercial quantities. One method, which recently has attracted a great deal of attention, involves the selective dehydrogenation of these long-chain, normal paraiiin hydrocarbons by contacting them and hydrogen with a nonacid, alumina-supported, platinum metal-containing catalyst. The principal feature of this method that is chiefly responsible for its commercial popularity relates to the ability of this catalyst system to perform the desired conversion to normal monoolefins with minimum interference from side reactions such as skeletal isomerization, dehydrocyclization, cracking, polymerization, secondary dehydrogenation, etc. In concrete terms, this feature means the ability to sustain relatively high levels of conversion at high selectivity for extended periods of time. However, I have now determined that this high selectivity feature of this preferred method is substantially reduced when a Wide-boiling feed stream is charged thereto. More specifically, the results obtained when charging a high boiling feed stream to this preferred process, relative to those that are obtained with a narrow boiling feed stream at a comparable weight percent conversion level, indicate a substantial loss of overall selectivity in the former case with an attendant increase in side products. The term wide-boiling feed stream is used herein to refer to a feed stream containing at least 4 normal parrafiin homologs each of which has at least 6 carbon atoms, and, conversely, a narrow boiling feed stream contains at the most '2 adjacent normal paraffin homologs.

The reasons why it is desired to charge a wide boiling feed stream to this preferred dehydrogenation method are determined largely by the nature of the final products that are to be made from the resulting normal monoolefins. A good example is given by the situation where an arylalkane detergent intermediate is to be produced from these long-chain normal monoolefins. In this case, the preferred procedure for adjusting the solubility, detergency, and the like properties of the final detergent, in-

ice

volves using a normal monoolefin source for the linear alkyl group on this intermediate, which source is wide enough in boiling point to incorporate at least 4 monoolefin homologs. Another reason for charging a wide boiling feed stream involves the ready availability of this type of stream and its relatively low cost. Accordingly, there is a substantial need for a selective normal parafiin dehydrogenation process that can handle a wide-boiling range feed stream with high selectivity, and I have now found such a process.

Without the intention of being limited by this explanation, I have determined that the observed degradation in selectivity and stability which attends the charging of a wide boiling feed stream to this preferred method, involves two principal considerations: one being the fact that the higher molecular weight normal paraffins dehydrogenate more readily than the lower homologs, and the second being a rather sharp decrease in selectivity, calculated on an individual homolog basis, that accompanies operation at conversion levels, based on each individual homolog, above about 12 wt. percent. Taken together, these two factors cause the observed selectivity degradation when a wide boiling feed stream is charged to the preferred catalyst system at conditions designed to achieve an overall conversion level of about to wt. percent, based on the feed stream, because at these conditions the higher homologs dehydrogenate much more readily and produce a conversion level, calculated on an individual homolog basis, that is above the level at which the selectivity for the corresponding normal monoolefins remains high. For example, for a C to C charge stock at conditions suificient for an overall conversion of about 10 to 11 wt. percent, the C and C homologs undergo about 15 wt. percent conversion whereas the C and C homologs undergo about a 7 to 8 wt. percent conversion; and, for these C and C homologs, at this conversion level there is a marked decrease in selectivity for the corresponding C and C monoolefins, leading to a sharp decrease in over-all selectivity and to process instability.

Regardless of theoretical considerations, I have now found that a wide-boiling, normal paraffin-containing feed stream can be dehydrogenated at higher overall selectivity for long-chain normal monoolefins when the feed stream is fractionated into a plurality of fractions and each of these fractions are separately dehydrogenated at severity levels which are inversely related to the average carbon number of the normal paraflins contained therein. In addition, I have found that this last method materially reduces the rate of catalyst deactivation since it substantially decreases the rate of formation of side products, such as dienes, trienes, cyclic compounds, etc., which are known precursors of carbonaceous deposits on the catalyst.

It is, therefore, an object of the present invention to provide a process for dehydrogenating a wide-boiling, normal parafiin-containing feed stream at relatively high overall selectivity and stability.

In one embodiment, the present invention relates to a catalytic process for dehydrogenating a hydrocarbon stream containing at least 4 normal parafiin homologs each of which has at least 6 carbon atoms. This process comprises the steps of: (a) separating the hydrocarbon stream into a plurality of fractions each of which contains at least one normal paraffin homolog; (b) separately dehydrogenating each of said fractions by contacting each fraction and hydrogen with a catalyst containing a platinum group component, an alkali component, and an alumina component, at a severity level for each fraction, which is inversely related to the average carbon number of the normal paraffin homologs contained therein, to produce a corresponding plurality of efiluent streams containing normal monolefins, unreacted normal paraffins and hydrogen; and, (c) withdrawing the effluent streams,

separating hydrogen therefrom, and recovering the resulting normal monoolefins.

In another embodiment, the invention comprises a process as outlined above wherein the catalyst contains an additional component selected from the group consisting of arsenic, antimony, bismuth, sulfur, selenium, tellurium, and compounds thereof.

A preferred embodiment involves the process as outlined above wherein the recovery of the resulting normal monoolefins is accomplished by combining the withdrawn effluent streams and charging the combined stream in admixture with an alkylatable aromatic to an alkylation zone containing an acid-acting alkylation catalyst at conditions suflicient to form arylalkanes, and recovering the resulting arylalkanes in a subsequent fractionation system.

Other embodiments and objects of the present invention encompass further details about: suitable catalysts for use in the dehydrogenation steps thereof, the feed streams that can be charged thereto, the process condiions used in each step thereof, the mechanics of the product recovery step associated therewith, preferred procedures for recycling unreacted normal parafiins recovered therefrom, etc. These embodiments and objects will become evident from the following detailed discussion of each of these elements of the present invention.

The hydrocarbon stream that is charged to the process of the present invention contains at least 4 normal paraffin homologs all of which have at least 6 carbon atoms. An especially preferred feed stream contains long-chain normal paraffins having about 9 to about 20 carbon atoms. In one embodiment wherein the resulting normal monoolefins are transformed into arylalkanes, useful as an intermediate in the manufacture of detergents, a hydrocarbon stream containing normal parafiins of about 10 to about 15 carbon atoms is quite commonly charged since these produce intermediates which can be utilized to make detergents having superior biodegradability and detergency. For example, a hydrocarbon stream contaming 4 or 5 homologs such as a C to C a C to C or a C to C fraction provides an excellent charge stock when detergent alkylate is to be produced. It is generally preferred that the amount of nonnormal hydrocarbons present in this feed stream be kept at low levels. Thus, it is preferred that the stream contain greater than wt. percent normal paraffins, with best results achieved at purities in the range of 96 to 98 wt. percent or more. It is within the scope of the present invention to pretreat the hydrocarbon feed stream via a suitable means for removing aromatic and cyclic compounds therefrom; for example, by contacting the feed stream with a solution of sulfuric acid, followed by a suitable neutralization operation. In a preferred embodiment, the feed stream is obtained by subjecting a hydrocarbon distillate containing normal parafiins in admixture with non-normal hydrocarbons within the desired boiling range to a separation operation employing one or more beds of molecular sieves which, as is well known, have the capability to produce extract streams having a very high concentration of normal components. A preferred separation system for accomphshing the production of a suitable feed stream is adequately described in US. Patent No. 3,310,486 and reference may be had thereto for the details about the mechanics of this type of separation. For example, a preferred procedure would involve charging a kerosene fraction boiling within the range of about 300 F. to about 500 F. to the separation system of the type described in US. Patent No. 3,310,486 and recovering therefrom a hydrocarbon stream containing a mixture of normal paraffins in the C to C range. Typically this last procedure can be performed so that the extract stream produced therefrom contains 98 wt. percent or more of normal parafiin hydrocarbons boiling within the range of 300 F. to about 500 F. It is, of course, understood that the above merely indicates a preferred source of the feed streams for the present invention; and any other suitable source of feed streams of the kind described may be used if desired.

It is an essential feature of the present invention that the catalyst used in the dehydrogenation steps thereof contain an alumina component, a platinum group component, and an alkali component. It is to be noted that the phrase alkali component is intended to include within its scope both alkali metals and alkaline earth metals and compounds thereof. Although it is not essential, it is generally preferred that the catalyst used in these dehydrogenation steps contain an additional component selected from the group consisting of arsenic, antimony, bismuth, sulfur, selenium, tellurium, and compounds thereof.

The alumina component generally has an apparent bulk density less than about 0.5 :gm/cc. with a lower limit of about 0.15 gm/cc. The surface area characteristics are such that the average pore diameter is about 20 to about 300 Angstroms; pore volume is about 0.10 to about 1.0 ml./gm.; and the surface are-a is about 100 to about 700 m.*/ gm. It may be manufactured by any suitable method including the well-known alumina sphere manufacturing procedure detailed in U.S. Patent No. 2,620,314.

The alkali component is selected from both alkali metals-cesium, rubidium, potassium, sodium, and lithium and the alkaline earth metals-calcium, magnesium, strontium, and barium. The preferred component is lithium. Generally, the alkali component is present in an amount, based on the elemental metal, of less than 5 wt. percent of the total catalyst with a value in the range of about 0.01 to about 1.5 wt. percent generally being preferred. In addition, the alkali component may be added to the alumina component in any suitable manner with impregnation by aqueous solutions being especially preferred. For example, an aqueous solution of lithium nit-rate provides an excellent impregnation solution.

The platinum group component is selected from the group of palladium, iridium, ruthenium, rhodium, osmium, and platinum, with platinum giving best results. This component is used in a concentration, calculated as an elemental basis, of 0.05 to about 5.0 wt. percent of the catalyst. This component may be used in any suitable form particularly including the oxides, sulfides, and other suitable compounds with best results generally obtained when this component is predominantly in an elemental metallic state. This component may be incorporated in the catalyst in any suitable manner with impregnation by water soluble compounds such as chloroplatinic acid being especially preferred.

The preferred fourth component is selected from the group consisting of arsenic, antimony, bismuth, sulfur, selenium, tellurium, and components thereof. Arsenic is particularly preferred. This component typically is used with good results in an amount of about 0.01% to about 1.0% by weight of the final composite. Moreover, this component is typically present in an atomic ratio to the platinum group component of about 0.1 to about 0.8 with intermediate concentrations being preferably employed such that the atomic ratio is about 0.2 to about 0.5. This component can be composited in any suitable manner with a particularly preferred method being impregnation by a water soluble solution of a compound such as arsenic pentoxide, etc.

This catalyst is typically subjected to one or more conventional drying and calcination treatments during its production. Additional details as to suitable dehydrogenation catalysts for use in the present invention, and methods of preparation, are given in the teachings of U.S. Patent Nos. 2,930,763; 3,291,755; and 3,310,599. Moreover, it is a good practice to presulfide the catalyst by any of the available techniques prior to its use in the present invention.

The conditions utilized in the dehydrogenation steps of the present invention are preferably quite similar except for the conversion temperature maintained in the respective steps. That is, it is preferred to adjust the severity level in the respective dehydrogenation steps by controlling the conversion temperature, although it is within the scope of the present invention to use any other means to vary the severity level. Accordingly, the pressure utilized in each of the steps is within the range of about 10 p.s.i.g. to about 100 p.s.i.g., with best results obtained in the range of about 15 p.s.i.g. to about 40 p.s.i.g. In addition, a liquid hourly space velocity of about 10 to about 40 hrs:- is preferably utilized in each of these steps; and it is preferred to use a hydrogen diluent in order to control the rate of formation of carbonaceous deposits on the catalyst and to provide a convenient source of heat for the endothermic reaction. Generally, the hydrogen is utilized in an amount such that the ratio of moles of hydrogen to moles of hydrocarbon charged to each of the steps is about 1:1 to about 20:1 with best results obtained when a ratio of about 5:1 to about 15:1 is used. Likewise, it is preferred to add a minor amount of water to each of the dehydrogenation steps in amounts suflicient to maintain the amount of water entering each of the dehydrogenation zones at a level of about 1000 p.p.m. to about 5000 p.p.m. or more based on the weight of the hydrocarbon stream being charged to the respective zone.

Regarding temperatures utilized in the dehydrogenation steps of the present invention, it is an essential feature of the invention that the temperature utilized varies inversely with the average carbon number of the normal paraffins contained in the fraction charged thereto. Viewed in another way, this essential feature of the present invention requires that the temperature of conversion, selected for each of the fractions into which the feed stream is divided, be based on the boiling point range of the fraction: that is a relatively low conversion temperature for a high boiling fraction; and, conversely, a relatively high temperature for a low boiling fraction. Hence, the conversion temperatnre used in each of the dehydrogenation steps of the present invention is selected, according to the above principle, from a range of about 750 F. to about 1000 F. As an example of the correlation of conversion temperature with the carbon number of the normal paraflin homolog in the fraction being charged to the particular dehydrogenation step, consider the information in the following table, which is derived from work which was performed with a preferred catalyst containing, on an elemental basis, 0.75 wt. percent Pt, 0.03 wt. percent As, 0.48 wt. percent Li, supported on an alumina carrier material, at a conversion level of 11.5 wt. percent, a LHSV of 32, 20 p.s.i.g., an H to HC ratio of 8:1, and a selectivity for the corresponding normal monoolefin of approximately Table I.Conversion temperature as a function of carbon It is, of course, understood that these data are only representative of this conversion level and these conditions, and other analogous correlations must be established for different conversion levels and different conditions. The essential point is that the severity level, determined principally by temperature, is to be chosen as an inverse function of the average carbon number in each of the fractions in order to avoid the selectivity degradation and catalyst deactivation problems which have been hereinabove discussed in detail. For example, for a C to C feed stream, the present invention requires the division of the feed stream into at least two fractions, a C and C fraction and a C and C fraction, with the C and C fraction being initially dehydrogenated at a relatively lower temperature of about 845 F. to about 850 F. and the C and C fraction being dehydrogenated at a relatively higher temperature of about 856 F. to about 862 F. It is to be noted that it is within the scope of the present invention to maintain the target conversion level by increasing the temperatures in the respective dehydrogenation steps; nevertheless, the difference between the temperatures used in each of the steps is maintained relatively constant in order to continuously produce improvements in overall selectivity and stability.

According to the present invention, the first step of the process is a separation step wherein the feed stream is separated into a plurality of fractions. This separation step may be performed in one or more conventional fractionating means, the details of which are well known to those skilled in the art and need not be repeated here. The essential point is that the fractionation must be made into a series of fractions each of which contains at least one normal paraffin homolog. In the preferred case each of these fractions contains no more than 2 adjacent normal paraffin homologs, although an improvement in overall selectivity and stability is obtained by any division of the feed stream, for example, into a higher boiling fraction and a lower boiling fraction.

The second step of the process then involves separately dehydrogenating each of the fractions obtained from the first step. As indicated above, these dehydrogenation steps all involve separately contacting the hydrocarbon fractions and hydrogen with the preferred catalyst at dehydrogenation conditions suflicient to produce normal monoolefins corresponding to the normal paraffins contained in each of these fractions. The conditions to be utilized in each of these steps have been given above, and it is only necessary to reemphasize here that the distinguishing feature of each of these separate dehydrogenation steps involves the severity level used in the respective dehydrogenation zone.

Following the dehydrogenation steps, a series of effluents containing the normal monoolefins produced in the dehydrogenation steps, unreacted normal paraffins, and hydrogen are withdrawn from each of the dehydrogenation steps, hydrogen is separated therefrom, and the normal monoolefins are recovered. The scope of this recovery step is intended to embrace a wide number of alternatives, particularly in regards to the means used to accomplish the monoolefin recovery and whether or not the elfiuent streams from each of the dehydrogenation zones are combined prior to being charged to the recovery step or whether separate recovery steps are used for each of these efiiuents.

Suitable physical means for recovery of the normal monoolefins involve the use of an adsorbent material having a high selectivity for the normal monoolefin such as activated silica gel, in the particle form, activated charcoal, activated alumina, various types of molecular sieves, and other adsorbents well known to those skilled in the art. Another physical means for performing this recovery step involves the use of an extraction solution having a high degree of selectivity for the normal monoolefins in a liquid-liquid extraction process or in an extractive distillation process. Among the chemical means that can be used to perform this separation are those that involve the selective reaction of the monoolefins to form a higher molecular weight product which then can be easily separated from unreacted normal paraffins by conventional fractionation means. A resume of suitable reactions for use in the recovery step is given above in conjunction with a discussion of the uses of the normal monoolefins and is not repeated here. The preferred procedure for recovery of the normal monoolefins involves the alkylation of an alkylatable aromatic and this procedure is explained in conjunction with the attached drawing.

As mentioned above, there are two principal alternative flow schemes for the recovery step of the present invention. One flow scheme involves: combining the efliuent streams from the dehydrogenation steps and passing the resulting mixture of the effluents to a single hydrogen separation zone. In this zone, a hydrogen-rich gaseous phase is separated from a liquid phase containing a mixture of the normal mono-olefins and unreacted normal paraffins. This last mixture is then withdrawn from this separating zone and subjected to one of the recovery means outlined above to separate the normal monoolefins therefrom and to produce a stream containing the unreacted normal paraffins. Preferably, this last stream is then recycled to the separating step.

A second flow scheme involves the use of a separate hydrogen separation zone and monoolefin recovery means for each effluent stream to produce a plurality of unreacted normal paraffin-containing streams corresponding to the plurality of effluent streams. Each of these unreacted normal paraffin-containing streams is then recycled to the corresponding dehydrogenation step. More particularly, this last alternative involves a separate train of hydrogen separation zone and a monoolefin recovery means for each of the efiluents from the dehydrogenation steps.

Regarding the attached drawing, it is understood that it merely represents a preferred embodiment of the present invention with no intent to give details about heaters, condensers, pumps, compressors, valves, process control equipment, and other conventional components, except where a knowledge of these devices is essential to the understanding of the pr sent invention or would not be self-evident to one skille l iwle art. Moreover, the following discussion of the attac ed drawing is given with reference to a particular feed stream in order to provide a specific example of the benefits which are derived from the present invention.

Referring now to the drawing, a hydrocarbon stream enters the process through line 1. In this particular case, this stream contains essentially 4 normal paraffin homologs boiling in the C through C range. Specifically, the feed stream contains 26.4 wt. percent n-Cn, 31.2 weight percent n-C 25.3 Wt. percent n-C and 13.3 'wt. percent n-C12 25.3 wt. percent n-C and 13.3 components and trace amounts of n-C and n-C Moreover, its initial boiling point is about 354 F., its 50% point 413 F. and its end boiling point 459 F. Just prior to the entrance of the charge into fractionation zone 2, at the junction of line 18 with line 1, it is commingled with a normal paraffin-containing recycle stream, the source of which will be explained below. The resultant mixture is then charged to fractionation zone 2, which in this case consists of a conventional fractionation means designed to separate the influent stream into a C and C cut which is taken overhead, and a C and C out which is recovered as bottoms. In this case the cut point is about 410 F. Accordingly, the drawing illustrates the case where a 4 homolog-containing feed stream is separated into 2 fractions each of which contains 2 adjacent normal paraffin homologs.

Following the fractionation step, the overhead stream passes through line 3 to first dehydrogenation zone 5. Prior to its entrance into zone 5 this stream is commingled with hydrogen, at the junction of lines 3 and 10, in an amount sufiicient to provide about 8.0 moles of hydrogen per mole of hydrocarbon contained therein. The resulting mixture is then heated, by conventional heating means (not shown in the attached drawing), and the heated mixture is passed into dehydrogenation zone 5. Zone 5 contains a fixed bed of spherical particles of an alumina catalyst containing about 0.75 wt. percent platinum, about 0.5 wt. percent lithium, and about 0.05 wt. percent arsenio-all calculated on an elemental basis. This catalyst is prepared according to the method given in US. Patent No. 3,291,755.

Zone is operated at the following conditions: an outlet pressure of 20 p.s.i.g., a liquid hourly space velocity of 32 hrr and an initial conversion temperature of about 860 F.

The resulting eflluent stream from zone 5 is withdrawn via line 7, cooled to a temperature of about 100 F. in a cooling means (not shown), and passed into hydrogen separating zone 9. An analysis of the stream leaving zone 5 via line 7 shows that it contains C and C normal monoolefins, unreacted C and C normal parafiins, hydrogen, and nonnormal components. Calculations based on this analysis show that the selectivity for the n-C and n-C normal monoolefins is about 91% and that the overall conversion is about 11.5 wt. percent.

Returning to the bottom stream produced in fractionation zone 2, it leaves this zone via line 4, and is commingled with about 8 moles of hydrogen per mole of hydrocarbon contained therein at the junction of line 10 with line 4. The resulting mixture is then heated and passed into second dehydrogenation zone 6. This zone contains a catalyst which is identical to that present in zone 5. Likewise, this zone is operated at a pressure of 20 p.s.i.g., and a liquid hourly space velocity of 32. The principal difference between the operation of zones 5 and 6 is the initial conversion temperature which is maintained at about 847 F. for zone 6 which is in sharp contrast with the temperature of about 860 F. used in zone 5.

An effluent stream is then withdrawn from zone 6, via line 8 cooled to a temperature of about 100 F., and passed into separating zone 9. An analysis of a sample of the stream in line 8 shows that the C and C normal paraffins are being converted, at a conversion level of about 11 wt. percent and at a selectivity for C and C normal monoolefins of about 92%.

In separating zone 9 a hydrogen-rich gaseous phase separates from the hydrocarbon-rich liquid phase. The gaseous phase is withdrawn via line 10, and a portion of this gaseous stream is vented from the system via line 19 in order to maintain pressure within the dehydrogenation zones. The remainder of this hydrogen-rich stream is recycled via line 10, through compressive means (not shown in the drawing) to the dehydrogenation zones as previously indicated.

The hydrocarbon-rich phase in separating zone 9 is withdrawn via line 11, commingled with a stream containing an alkylatable aromatic at the junction of line 11 with line 16 and passed into alkylation zone 12. In this example, the alkylatable aromatic is benzene, and it is used in a mole ratio of about 12 moles of benzene per mole of normal monoolefin contained in the hydrocarbon stream flowing through line 11.

In alkylation zone 12, the reactants are maintained in contact with substantially anhydrous hydrogen fluoride for a reaction period of about 5 to about 30 minutes. The anhydrous hydrogen fluoride is used in a volumetric ratio of about 0.5 to about 2 volumes of hydrogen fluoride solution per volume of the total hydrocarbon stream entering the zone. In addition, alkylation zone 12 is maintained at a temperature of about 70 F. to about 150 F., a pressure sufficient to maintain reactants and catalyst in liquid phase, and means are provided within alkylation zone 12 to remove the heat produced by the exothermic alkylation reaction. The hydrocarbon products of the alkylation reaction are then separated in a settler vessel within zone 12 from the hydrogen fluoride phase to produce a hydrocarbon effiuent stream which is withdrawn from zone 12 via line 13 and passed to fractionation system 14.

In general, fractionation system 14 can comprise any suitable train of fractionation means designed to separate the hydrocarbon stream charged thereto into a benzenerich fraction, an unreacted normal paraflin-rich fraction, :1 phenyl-substituted normal paraifin-containing fraction and a heavy alkylate fraction. Preferably, this system comprises three fractionation columns: the first column operating on the input stream from line 13 to produce overhead a benzene-rich fraction, which is recycled to alkylation Zone 12 via lines 15 and 16, and a bottoms fraction; the second column operating on the bottoms fraction from the first column to form an unreacted normal paraflin-containing stream, which is recycled via line 18 to fractionation zone 2, and a bottoms fraction; and the third column operating on the bottoms fraction from the second column to produce an overhead comprising phenyl-substituted C to C normal paraffins, which is recovered via line 17, and a minor amount of heavy al kylate as bottoms, which is removed via line 20.

Operation is continued as outlined above for a process period of about 100 barrels of feed stream entering through line 1 per pound of total catalyst utilized in zone 5 and zone 6, and throughout this period the conversion temperatures utilized in zone 5 and zone 6 are continuously adjusted in order to maintain an over-all conversion level of 11.5 wt. percent based on the combined feed to fractionation zone 2. It is found that the catalyst employed in zone 5 and zone 6 deactivates at a similar average rate of about 005 F. per barrel of charge to each zone per pound of catalyst contained in each zone. Accordingly, the difference in severity levels employed in the respective zones is maintained substantially constant during this run. In addition, an analysis of the yield of phenyl-substituted C to C normal paraffins recovered over this period via line 17 indicates that the average selectivity of the process for C to C normal monoolefins is about 90%. These results stand in sharp contrast with the results obtained when the feed stream is charged directly to a single dehydrogenation zone containing an equal amount of an identical catalyst and at conditions selected to produce a per pass conversion level of 11.5 wt. percent. In this latter case, the catalyst is found to deactivate at a rate of about 1.5 F. per barrel of charge per pound of catalyst, and the average overall selectivity is about Accordingly, the advantages of the present invention are evident.

I claim as my invention:

1. A catalytic process for dehydrogenating a hydrocarbon stream containing at least 4 normal paraffin homologs each of which has at least 6 carbon atoms, said process comprising the steps of:

(a) separating said hydrocarbon stream into a plurality of fractions each of which contains at least one normal paraflin homolog;

(b) dehydrogenating each of said fractions by separately contacting each fraction and hydrogen with a catalyst containing a platinum group component, an alkali component, and an alumina component, at a severity level, which for each fraction is inversely related to the average carbon number of the normal paraflins contained therein, to produce a corresponding plurality of effiuent streams containing normal monoolefins, unreacted normal paraflins, and hydro- (0) withdrawing the resulting efl'luent streams, separating hydrogen therefrom, and recovering the resulting normal monolefins.

2. The process of claim 1 wherein said catalyst contains a component selected from the group consisting of arsenic, antimony, bismuth, sulfur, selenium, tellurium, and compounds thereof.

3. The process of claim 2 wherein said catalyst is a composite of alumina, about 0.01 to about 1.5 wt. percent lithium, about 0.05 to about 5.0 wt. percent of platinum, and arsenic in an amount of about 0.1 to about 0.8 atom of arsenic per atom of platinum.

4. The process of claim 1 wherein said hydrocarbon stream contains normal paraflin hydrocarbons boiling in the C to C range and said separation is into a first fraction containing C and C hydrocarbons and a second fraction containing C and C hydrocarbons.

5. The process of claim 1 wherein: the efiluent streams said last streams is separately recycled to combine with the corresponding fraction prior to subjection of said fraction to said dehydrogenation step.

7. The process of claim 1 wherein said separating step is operated to produce a plurality of fractions each of which contains 2 adjacent normal paraffin homologs.

8. The process of claim 1 wherein the recovery of said resulting normal monoolefins is effected by combining said efiluent streams and charging the resulting combined stream in admixture with an alkylatable aromatic hydrocarbon to an alkylation zone containing an acid-acting alkylation catalyst at conditions forming arylalkanes, and recovering the resulting arylalkanes by subsequent fractionation.

References Cited UNITED STATES PATENTS 2,878,179 3/1959 Hennig 208144 3,002,916 10/ 1961 Hamilton 208-93 3,047,490 7/ 1962 Myers 208-141 3,072,561 1/ 1963 Cahn 208- 3,072,562 1/ 1963 Bowles 208-93 3,124,523 3/1964 Scott 20895 3,126,426 3/1964 Turnquest et a1. 260-6833 3,168,587 2/1965 Michaels et al 260-6833 3,293,319 12/1966 Haensel et al. 260-6833 3,360,586 12/1967 Bloch et a1 260-683.3

OTHER REFERENCES K. K. Kearby: Dehydrogenation of Paraffins, pp. 454-456 of chapter 10 in Catalysis, volume 3, P. H. Emmett, editor, Reinhold Pub. Corp. (1955).

DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant Examiner US. Cl. X.R. 

